Seek-scan probe (SSP) memories are a type of memory that uses non-volatile storage media as the data storage mechanism and offers significant advantages in both cost and performance over conventional charge-storage memories. Typical SSP memories include storage media made of materials that can be electrically switched between two or more states having different electrical characteristics such as resistance, polarization dipole direction, or some other characteristic.
SSP memories are written to by passing an electric current through the storage media or applying an electric field to the storage media. Passing a current through the storage media, or applying an electric field to the media, is typically accomplished by applying a voltage between a sharp probe tip on one side of the storage media and an electrode on the other side of the storage media. Current SSP memories use probe tips positioned on a free end of one or more MEMS probes. In an idle state each MEMS probe maintains the probe tip at a certain distance from the storage media, but before the electric field or current can be applied to the storage media the probe tip must usually be brought close to, or in some cases in direct contact with, the storage media.
FIGS. 1A-1C illustrate tracking in a common SSP memory configuration. FIG. 1A illustrates an SSP memory configuration in which a cantilever probe is anchored to a substrate (the cantilever wafer), and can be actuated to contact or de-contact the storage media on a mover that carries a storage media and is positioned over the cantilever wafer. The data tracks are stored in the storage media in one of two ways, depending on how the media mover scans relative to the cantilever tips. FIG. 1B illustrates axial scanning, where data is stored in the storage media in-line with the cantilever direction, such that the mover scans in the direction parallel to a longitudinal axis of the cantilever to read/write/erase (R/W/E) each data track. FIG. 1C illustrates transverse scanning, where the media mover scans in a direction perpendicular to the longitudinal axis of the cantilever probe to R/W/E each data track; data is consequently stored in lines that are transverse to the cantilever's longitudinal axis.
To maximize the amount of data that can be written in the storage media the data density should be very high, meaning that the data tracks in the storage media—whether axial or transverse—can be very close together (e.g., <20 nm). During axial or transverse media mover data scanning, the data track will drift differently due to various factors such as temperature change from environment. Furthermore, the localized temperature gradient generated by surrounding electronics can also cause significant data track drift. As a result the cantilever could perform incorrect data streaming on the wrong tacks (e.g., it could intend to R/W/E on track #1, but instead R/W/E on track #3 instead of track #1) and thus cause data errors. Since the mover can only correct the scanning globally and the cantilever can only be actuated vertically, these two elements can be used for only very limited and localized correction of track error. In a worst case, only one cantilever can be used at a time. This will drastically limit the data rate and thus result in very poor device performance.